Abstract: An anti-jackknifing control device for a tractor-trailer which includes a fifth wheel having a pair of oppositely disposed pneumatically operated cylinder-piston assemblies, the pistons of which are insertable into arcuate slots in a bottom plate on the trailer so as to limit the turning movement of the tractor relative to the trailer to a predetermined angle in either direction from a centerline corresponding to a longitudinally aligned position of the tractor-trailer together with an alarm in the driver's cab actuted by a limit switch in each of the slots which is closed by the bottom plate at a turning angle in either directon intermediate the centerline and predetermined angle.